1. Field of the Invention
The present invention relates to a photographic color coupler, more particularly, it relates to a novel class of two-equivalent color couplers, a color photographic light-sensitive material containing such color couplers and a method of forming images using such color couplers.
2. Description of the Prior Art
It is well known that an oxidized aromatic primary amine developing agent reacts with a dye forming coupler to form a color image by color development after exposure of a silver halide photographic light-sensitive material to light. In this method, a color reproduction process according to a conventional subtractive method is applied to form a cyan, magenta or yellow color image which is in a complementary relationship with red, green or blue color. For instance, a phenol derivative or a naphthol derivative is used as a coupler for forming a cyan color image, a 5-pyrazolone derivative, a pyrazolobenzimidazole derivative or a indazolone derivative is used for forming a magenta color image, and a pivaloylacetanilide derivative or a benzoylacetanilide derivative is used for forming a yellow color image.
In a color photographic process, a color forming coupler which is added to a developer solution or incorporated in a light-sensitive photographic emulsion layer or other color image forming layer reacts with an oxidation product of a color developing agent formed upon development to form a non-diffusible color image. Almost all conventional couplers are four-equivalent couplers, that is, theoretically, they stoichiometrically require four mols of exposed silver halide as an oxidizing agent for the formation of one mole of a dye. Incorporation of a large amount of silver halide in a light-sensitive layer is disadvantageous because the sharpness of the images formed deteriorates due to increased light scattering in the emulsion layer or because the rate of processing the light-sensitive material is reduced due to increased thickness of the emulsion layer. Further, the formation of dyes by these four-equivalent couplers has the disadvantage that a strong oxidizing agent must be employed in a subsequent processing step, because the complete formation of the dyes does not occur in a color development bath.
In order to overcome these disadvantages of two-equivalent couplers, that is, couplers which require only two mols of exposed silver halide for forming one mol of a dye, has been proposed.
Two-equivalent couplers have a structure in which one hydrogen atom of the coupling position, such as the para-position of a phenolic hydroxy group, an active methylene group at the 4-position of a 5-pyrazolone or an active methylene group of an acylacetanilide, is substituted with a releasable group. Examples of releasable groups are a sulfonamido group as described in U.S. Pat. No. 3,737,316, an imido group as described in U.S. Pat. No. 3,749,735, a sulfonyl group as described in U.S. Pat. No. 3,622,328, an aryloxy group as described in U.S. Pat. No. 3,476,563, an aryloxy group as described in U.S. Pat. No. 3,419,391, a thiocyano group as described in U.S. Pat. Nos. 3,214,377 and 3,253,924, a carbamoyl group as described in Japanese Pat. application (OPI) 74539/1974, an imido group as described in Japanese Pat. application (OPI) 53436/1974, an aminosulfonyloxy group as described in Japanese Patent Publication 12661/1974 and a sulfinyloxy group as described in Japanese Pat. Publication 12660/1974.
However, these known couplers are not always satisfactory because they have the disadvantages that coupling reactivity is insufficient, dispersibility is inferior, resulting in difficulties in coating, remarkably high color fog is produced, the synthesis of the couplers is very difficult, the storability of the color images formed is poor, and the like.